Interferon (IFN) induction and signaling is an early antiviral response triggered by viral double-stranded (ds)RNA. While critical to limiting viral replication and spread, overexpression of the IFN response can be detrimental to human health. For example, Aicardi-Goutires syndrome (AGS) is a severe neurodevelopmental and inflammatory genetic disease, characterized by excessive IFN production and signaling induced by host self-dsRNA. Thus it is important to understand how the host regulates dsRNA pathways induced by self- dsRNA as well as viral (non self)-dsRNA. Oligoadenylate synthetase-ribonuclease L (OAS-RNase L) is a potent antiviral pathway that severely limits pathogenesis of many viruses. Upon sensing dsRNA, OASs produce 2',5'-oligoadenylates (2-5A) which activate RNase L to cleave both host and viral single-stranded RNA thereby limiting protein production, virus replication and spread, and leading to apoptotic cell death. During the last period we identified and characterized virus- and host-encoded phosphodiesterases (PDEs) as potent antagonists of RNase L activation, most notably the PDE of murine coronavirus (MHV), NS2, a liver specific virulence factor. We also identified OAS3 as the principal antiviral OAS in human cells. Moreover, we found that RNase L is the dominant dsRNA-dependent pathway leading to apoptosis in human cell lines by exogenous dsRNA or by self-dsRNA in cells ablated for expression of ADAR1, an enzyme that destabilizes dsRNA and when mutated can cause AGS. However, there is a gap in understanding how the activation of RNase L is controlled to prevent the potentially destructive effects of dsRNA while still maintaining the ability to limit viral replication and spread. In the continuation of this project, we will test the hypothesis that the host and viruses have multiple pathways to control the levels of 2-5A, its activation of RNase L and the directly antiviral as well as proapoptotic and proinflammatory effects of RNase L. We will use innovative approaches to investigate the OAS-RNase L system in three complementary species-specific systems, each with unique features. We will investigate: 1) how ADAR1 isoforms and host PDEs regulate levels of 2-5A in human cells, and determine which OAS isoforms are involved; 2) how the antiviral, apoptotic and inflammatory roles of RNase L cooperate to effect viral clearance in an MHV mouse model; and 3) how the OAS-RNase L pathway is expressed and activated in bats, and the mechanism(s) underlying antagonism of OAS-RNase L activation by filovirus VP35. Results of these studies should provide a better understanding of how host enzymes control the levels of self-dsRNA and 2-5A to limit RNase L activity in uninfected cells, while allowing activation during viral infections as well as the combined antiviral, apoptotic and proinflammatory roles of RNase L in viral clearance. These studies will allow us to identify targets that enhance host cell resistance to virus while minimizing the threat of over activation of OAS-RNase L to the host, and finally may contribute to identifying therapeutic strategies for viral diseases and for AGS patients with mutations in ADAR1.